METHODS 175 



some places, the greater density of the vegetational cover 

 acted to limit the range of sight. But for each of these adverse 

 features of the method there were certain compensations." 

 One of the advantages of this method is that it gives a good 

 idea of the relative numbers of animals in any association, and 

 this is one of the most important types of fact about which 

 we require information. It seems probable that the method 

 will give information of great value, so long as a sufficient 

 number of censuses are obtained by different people, in order 

 to eliminate the effects of factors like the weather, time of day, 

 rate of travel, etc. 



16. In addition to censuses giving the average numbers 

 of animals in different habitats, we require methods of recording 

 the changes in numbers from month to month or year to year. 

 Of course, a series of censuses of the kind described will provide 

 this information, but in many cases there is not the time, staff, 

 or opportunity for carrying out censuses of sufficient accuracy, 

 in which the methods will remain the same as time goes 

 on, so that the results are comparable. The chief difficulty 

 of recording changes in the numbers of any animal which 

 undergoes violent fluctuations in numbers is in finding a 

 standard to which the abundance in different years can be 

 referred. Such statements as " wasps are more abundant 

 than usual " cannot be safely used, for two reasons. The 

 first is that we do not know what '' usual " means ; the second 

 is that its meaning varies from year to year, and in the minds 

 of different people. The latter is due to the fact that most 

 people do not remember with any accuracy for more than 

 about five years ; and also that more significance is attached to 

 recent years than to earlier ones. The result of all this is 

 that the word " usual " when applied to numbers may mean 

 practically anything, according to the particular emotions, 

 powers of observation, and strength of memory of the observer. 

 The records of butterfly abundance in England given in the 

 Phenological Reports of the Royal Meteorological Society, 

 prove conclusively and surprisingly that butterflies are " scarcer 

 than usual " in about one year out of every five ! It comes to 

 this, that records referring to the '* usual " are only of value 



